This invention relates to doctor blade systems and more particularly to doctoring in imaging systems utilizing liquid developers.
The formation and development of images on the surface of photoconductor material by electrostatic means is well known. The basic xerographic process as taught by C. F. Carlson in U.S. Pat. No. 2,297,691 involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light and shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic charge pattern image by depositing on the image a finely divided electroscopic marking material referred to in the art as "toner". The toner will normally be attracted to those areas of the layer which retain a charge thereby forming a toner image corresponding to the electrostatic charge pattern. The powder image may then be transferred to a support surface such as paper and permanently affixed to the support by any suitable means such as heat fixing or solvent fixing. Alternatively, the powder image may be fixed to the photoconductive layer if elimination of the powder transfer step is desired. In addition, instead of forming a charge pattern by uniformly charging a photoconductor followed by image-wise light exposure, a charge pattern may be formed by directly charging an insulating or a photoconductive layer in image configuration. Other methods are known for applying electroscopic particles to the imaging surface. Included within this group are the "cascade" development technique disclosed by E. N. Wise in U.S. Pat. No. 2,618,552; the powder cloud development technique disclosed by C. F. Carlson in U.S. Pat. No. 2,221,776; and the magnetic brush process disclosed for example in U.S. Pat. No. 2,874,063.
Development of a charge pattern image may also be achieved with liquid rather than dry developer materials. In conventional liquid development more commonly referred to as electrophoretic development an insulating liquid vehicle having finely divided solid material dispersed therein contacts the imaging surface in both charged and uncharged areas. Under the influence of the electric field associated with charged image pattern, the suspended particles migrate towards the charged portions of the imaging surface separating out of the insulating liquid. This electrophoretic migration of charged particles results in the deposition of the charged particles on the imaging surface in image configuration. Electrophoretic development of a charge pattern may, for example, be obtained by pouring the developer over the image surface, by emersing the image surface in a pool of the developer or by presenting the liquid developer on a roller and moving the roller against the imaging surface. The liquid development technique has been shown to provide developed images of excellent quality and to provide particular advantages over other development methods in offering ease in handling.
Automatic copying machines employing liquid development techniques generally can be divided into two categories. In the first, an electrophotographic sheet is developed and the sheet and developed image is used as final copy. In the second, an image is developed on a photoconductive surface and the image is subsequently transferred to a transfer sheet which forms the final copy with the photoconductive element being reused for subsequent copies.
A preferred method for developing electrostatic charge patterns is shown in British patent specification No. 880,597. In this system an applicator roll is utilized to present liquid developer to the surface of the member carrying the charge pattern. The amount of liquid on the applicator roll is carefully controlled by using a doctoring or metering blade. It has been found with this system that the quality of the final images produced depend greatly on temperature of the liquid developer in the system. It has been determined that variation in temperature causes a viscosity change in the liquid developer which affects the final developed image density. In general, at higher operating temperatures the reproduced images are more dense than at lower operating temperatures. This result can be explained when it is realized that at higher operating temperatures the liquid becomes less viscous and thus more readily transferable at the image developing station. Accordingly, to provide uniform results at an operating temperature of 10.degree. C. and at 40.degree. C. where the viscosity change can be as much as 90% requires a system for temperature compensation.